1. Field of the Invention
The present invention relates to a drive system for turbochargers with motor-generators mounted on their rotatable shafts, and more particularly to a drive system for a multistage turbocharger assembly with series-connected turbines and compressors.
2. Description of Prior Art
There are known turbine-driven turbochargers in which exhaust gases emitted from an engine are led to an exhaust turbine to rotate the turbine and a compressor coupled thereto at a high speed for supercharging the engine. Japanese Laid-Open Patent Publication No. 60-43152 discloses a system for recovering and feeding any remaining exhaust energy back to the shaft of the engine after the exhaust energy from the engine has been recovered by the exhaust turbine to drive the compressor. The disclosed system has a rotary electric machine and an intake air compressor which are mounted on the rotatable shaft of the exhaust turbine. The exhaust energy which is recovered as rotational energy by the exhaust turbine is used to rotate the compressor for supercharging the engine. At the same time, the rotary electric machine operates as an electric generator to generate electric energy which is supplied to an electric motor coupled to the shaft of the engine. The motor coupled to the engine shaft is rotated to assist in rotating the ending shaft, thereby feeding the exhaust energy recovered by the exhaust turbine back to the engine shaft.
When the engine rotates at a low speed and hence the exhaust energy is small, however, the above system can neither generate electric energy nor supercharge the engine. To avoid this shortcoming, it has been proposed a system in which when the engine rotates at a low speed, the rotary electric machine mounted on the rotatable shaft of the exhaust turbine is operated as a motor to rotate the compressor through the rotatable shaft for supercharging the engine (see Japanese Laid-Open Patent Publication No. 60-195329).
The system disclosed in the latter publication however requires large electric power to drive the rotary electric machine. Since such large electric power is supplied from a battery, the battery tends to run out of the stored electric energy quickly, and various other electric devices connected to the battery may not be supplied with a desired amount of electric energy. The battery is also apt to be of a short service life. Since only one turbocharger is mounted on the engine, the inertial mass of the rotating parts of the turbocharger is large, making the turbocharger less responsive to the operation of an accelerator pedal.